According to the National Osteoporosis Foundation ten million Americans have osteoporosis, and an estimated 34 million with low bone mass are at risk of developing osteoporosis (http)://www.nof.org/osteoporosis/diseasefacts.htm). Called the “silent disease,” OSP develops slowly over a number of years without symptoms. Eighty percent of those affected are women, particularly petite Caucasian and Asian women, although older men and women of all races and ethnicities are at significant risk.
In the United States, 700,000 people are diagnosed with vertebral compression fractures as a result of OSP each year. Morbidity associated with vertebral fractures includes severe back pain, loss of height and deformity, all of which negatively affect quality of life.
Once microfracture of the vertebra begins, there is little the clinician can do except palliative medical treatment using analgesics, bed rest and/or restriction of activity. With time, the microfractures widen at one level and without surgical intervention, the fractures cascade downward with increasing kyphosis or “hunching” of the back. Once a mechanical lesion develops, surgery is the only option. Vertebroplasty or kyphoplasty are the primary minimally-invasive surgical procedures performed for the treatment of compression-wedge fractures due to OSP.
Vertebroplasty stabilizes the collapsed vertebra by injecting polymethylmethacrylate (PMMA) or a substantially equivalent bone cement into cancellous bone space of the vertebrae. Besides providing structural support to the vertebra, the exothermic reaction of PMMA polymerization is said to kill off the nociceptors or pain receptors in the bone, although no proof of this hypothesis has been provided in the literature. This procedure is typically performed as an outpatient procedure and requires only a short-acting local or general anesthetic. Once the surgical area of the spine is anesthetized, the physician inserts one or two needles through small skin incisions into either the pedicle (uni-transpedicular) or the pedicles of the vertebral body i.e., bi-transpedicular. PMMA is injected through the needle and into the cancellous-bone space of the vertebra.
Kyphoplasty mirrors the vertebroplasty procedure but has the additional step of inserting and expanding a nylon balloon in the interior of the vertebral body. Expansion of the balloon under pressure reduces the compression fracture and creates a cavity. After withdrawal of the balloon, PMMA is injected into the cavity to stabilize the reduction. The kyphoplasty procedure may restore the vertebral body height. Kyphoplasty is an in-patient surgery that requires hospitalization and a general anesthetic. Kyphon Inc. claims over 275,000 spinal fractures have been treated using their PMMA derivative and their “balloon” kyphoplasty procedure worldwide (Sunnyvale, Calif., Sep. 5, 2006, (PR NEWSWIRE) Kyphon study 2006).
Bone cement for both vertebroplasty and kyphoplasty procedures currently employ variations of standard PMMA in a powder and a methyl methacrylate monomer liquid. When the powder and liquid monomer are mixed, an exothermic polymerization takes place resulting in the formation of a “dough-like” material, which is then inserted into the cancellous bone space. The dough, when hardened, becomes either the reinforcing structure or the grout between the bone and prosthesis.
The average clinical in vivo life of the PMMA grout is approximately 10 years due to corrosion fatigue of either the bone-cement/prosthesis and/or the bone cement/bone interfaces. Jasty et al. (1991) showed that in cemented total hip replacements: “Fractures in the cement mantle itself were found on cut sections around all prostheses which had been in use for over three years.” Jasty et al. also noted: “In general, specimens less than 10 years in situ showed small incomplete fractures while the specimens in place more than 10 years all showed large complete cement mantle fractures.”
When an implant fails, a revision becomes mandatory. After removal of the cement and hardware, a cemented arthroplasty can be repeated if enough cancellous bone matrix exists to grip the new PMMA. Alternatively, cement-less prosthesis can be installed. Such a revision, however, can only be applied to total joint replacement failures. For vertebroplasty and/or kyphoplasty, a classical screw and plate internal fixation with autograft fusion is necessary.
Despite advances in the foregoing procedures, there remains a need for improved bone cement delivery systems which enable rapid and controllable deployment of bone cement for the treatment of conditions such as vertebral compression fractures.